EQUATION OF STATE, BONDING CHARACTER, AND PHASE-TRANSITION OF CUBANITE, CUFE2S3, STUDIED FROM O-GPA TO 5-GPA

An in-situ study of cubanite, CuFe2S3, was performed in a diamond-anvi l cell using Mossbauer spectroscopy and energy-dispersive X-ray diffra ction at room temperature and pressures up to 5 GPa. Mossbauer spectra of orthorhombic cubanite show a single Fe site with rapid electron tr ansfer between Fe2+ and Fe3+, a hyperfine magnetic field that is relat ively insensitive to pressure, and a center shift that decreases with pressure because of increasing covalency. A phase transition occurs ab ove 3.3 GPa that involves a change from the orthorhombic cubanite stru cture to a derivative of the hexagonal NiAs (B8) structure, with a zer o-pressure volume decrease of 29%. The large difference in volume is c aused by a change from tetrahedral to octahedral coordination and a si gnificant shortening of metal-metal bonds. Volume-compression data wer e fitted to a second-order Birch-Murnaghan equation of state (K'(0) = 4) with the results K-0 = 64 +/- 3 GPa (orthorhombic phase) and K-0 = 157 +/- 16 GPa (high-pressure phase). Mossbauer data of the high-press ure phase indicate a single Fe site with no magnetic ordering and a va lence intermediate between Fe2+ and Fe3+. Consideration of likely orde ring patterns in the high-pressure phase indicates that localized elec tron transfer could occur along face-shared pairs of Fe octahedra, and extended electron delocalization could occur along paths formed by fa ce and edge-shared octahedra.